Devices to monitor temperature and other environmental variables affecting packaged goods during shipping or through the logistics chain are in widespread use. Most such devices are large, expensive and require the package to be opened to retrieve the environmental data from the device. Petersen and Wilson in Canadian Patent No. 2627258 have described a small low cost, disposable device (tag) for monitoring, recording and downloading the temperature history of the internal environment of a package in transit. Due to size and cost considerations the onboard computational power of such devices is limited. This has implications for their functionality.
In simple, disposable, low-cost temperature-monitoring devices on-board firmware typically converts sensor generated resistance values to temperature readings using the Hart-Steinhart approximation relating resistance to temperature for semiconductor thermistors. Due to the low computational power inherent in such low-cost tags, each conversion of a resistance value to a temperature value results in significant degradation of the tag's accuracy due to rounding errors. Current temperature monitoring devices convert thermistor-derived resistance values to temperature values in real time and either store the latter readings or display them directly as via an LCD. This results in compounding the rounding errors as a function of the number of conversions. Such devices are typically limited to accuracy of ±4° C.
A need has been identified for further refinements to such devices to increase the accuracy with which said device can monitor the environment, allow said device to communicate its environmental data history simply and without recourse to an external reader device and permit manual termination of said device's data acquisition function at any point during transit or in the logistics chain to save battery power or for other reasons. In addition, other needs include permitting automatic termination of said device's data acquisition function at any point during transit or in the logistics chain to save battery power or for other reasons, detecting unauthorized or surreptitious opening of the package during transit or in the logistics chain and permitting said device to operate in a food grade or pharmaceutical grade environment and to transmit its environmental history data without the package environment being disturbed as by opening. Further refinements include the ability to operate in environments whose temperatures are higher or lower than those at which most electronic battery powered devices fail or become unreliable (>100° C. or <−40 minus 40° C.) and broadcast by RF over greater range.